The need to monitor and control remote apparatus from a central control point is well known. One such need arises in the art of electric power distribution. Power from one or more generator locations is distributed by a network of distribution lines which extend to substations. Further distribution lines extend from each substation in order to reach all of the electric power customers in the service area and distribute power to them. Switching devices are interposed throughout the distribution system and service area to enable the system to be controlled by removal of one or more selected circuits from service in the event of fault or overload. Detection of the fault or overload at the central control point and commands sent therefrom via a communications path (typically provided by the telephone system) to cause actuation of a remotely actuable switch enable the control point operator to remove the fault area from the distribution network forthwith, thereby preventing further breakdown of the network on account of the fault condition.
One known approach for providing central monitoring and control of remote switching devices is set forth in U.S. Pat. No. 4,045,714 for "Remote Switch Control and Status Indicator System". The invention described in that patent provided a workable way to monitor circuit faults and operate remote power switches from a central location. A further improvement in that system is described in U.S. Pat. No. 4,128,856 entitled "Automatic Resetting Control Circuit for Air Switch Operator". Some pertinent background information may also be found in U.S. Pat. No. 4,520,234 for "Remote Cable Switching System".
The prior system used telephone audio tone signalling between the central control point and each wire center location. A metallic wire pair, usually provided by the telephone company, extended between each remotely controllable switch and its associated wire center. Constant current in a first flow direction and at a nominal voltage of 48 volts was applied over the wire pair, and the resultant voltage drop across the loop established the actual conditions at the remote site. Various resistance elements were selectably interposed in the loop in accordance with monitored conditions at the remote site and thereby affected the voltage drop across the loop. For example, if a fault condition was present, such as overcurrent, an overcurrent detector would trip, shunting across a resistance in the control loop. Another resistance or shunt was present, depending upon whether the remote switch was open or closed. The voltage drop was measured at the wire center and then converted into an analog signal which was sent to the operator at the central control point. The following table sets forth examples of the conditions at the remote site which were being monitored:
______________________________________ Voltage Condition Code ______________________________________ 0-5 Short circuit in wire pair. 0 8.25-13.25 Switch closed, fault condition present. 2 18.00-23.00 Switch closed, no fault. 4 28.75-33.75 Switch open, fault condition present. 6 38.50-43.50 Switch open, no fault. 8 45-50 Wire pair open. 5 ______________________________________
With the prior system the operator at the central control location could dial up a wire center via telephone and obtain the current status of any one or all of the remote switch sites to which the wire center was connected. In addition, the operator could signal remote actuation of the switch if it were provided with an automatic actuator.
In order for remote actuation of the switch to occur, the current loop for monitoring conditions was removed from the line, and a power supply of higher voltage, typically 100 volts or more (coin collect or return) was applied across the wire pair in reverse sense or polarity. This higher voltage of reverse polarity passed through a control circuit to charge an actuation capacitor of high storage capacity. Once the charging voltage was removed from the wire pair, the energy stored in the capacitor was applied to operate an actuation solenoid, and the switch was thereupon automatically moved from its closed position to its open position. This new condition, code 8, was then presented across the control loop, and the operator at the central facility could readily determine this new condition.
While the solutions proposed by the referenced prior patents worked well, they were manual insofar as monitoring activity via the wire center units was concerned, and their use resulted in a number of flaws and drawbacks. Since those systems relied upon the telephone network for the communications path, certain evolutionary changes in the telephone system have negatively impacted those prior solutions. For one thing, the original remote control circuits were designed to operate when voltage in excess of approximately 56 volts appeared across the tip and ring wires of the communications pair. As electronic telephone switching systems have become prevalent, the telephone central office battery voltage has gone up in value. When applied to a wire pair connected in reverse sense, this higher nominal line voltage has actually caused the remote switch to be operated, instead of being monitored, thereby inadvertently disconnecting service area customers from the electric power distribution network and resulting in customer complaints.
This problem of reversed wire pairs has led directly to another problem. Prior telephone maintenance practice and procedure has been to apply an ohmmeter across the tip and ring wires in order to sense in which direction (actuation polarity) the capacitor is seen to be charging, by deflection of the ohmmeter needle. The problem has been that telephone company ohmmeters have applied approximately 60 volts across the capacitor. The capacitor having charged to 56 volts now discharges and the switch is inadvertently opened, cutting off customers from electric power.
Another drawback lay in the fact that unless the central control operator had reason to monitor a particular switching unit, no regular monitoring access was made to each remotely located switching unit. During times of inclement or harsh weather conditions, deterioration of failure of the wire pair might go undetected until it was desired to operate the remote switch controlled by the switching unit. By then, it was too late, and a lineman had to be dispatched to operate the switch manually.